The invention relates to a method of operating an internal combustion engine in a compression ignition mode during low and medium load operation and in a positive or spark ignition mode in higher load ranges.
In a reciprocating piston internal combustion engine in each cylinder a longitudinally movable piston delimits a combustion chamber in which a mixture of fuel and oxygen-containing fresh gas is burnt. For the admission of the fresh gas and the discharge of the exhaust gases, inlet and outlet channels are provided which include gas exchange valves which are controlled by a valve drive for performing a cyclic charge change.
In an operating mode with positive or spark ignition, the air/fuel mixture formed in the combustion chamber is ignited by the spark of a spark plug projecting into the respective cylinder. With spark ignition according to the Otto principle, a stoichiometric air/fuel mixture is formed by throttling of the fresh air flow in the air inlet channel to provide the required mixture volume. There is also another engine operating mode known which is called compression ignition or space ignition wherein, by a certain increase of the combustion chamber temperature by compression of the air/fuel mixture in the combustion chamber, the air/fuel mixture is ignited. In this operating mode, in which the fresh air is supplied to the cylinder unthrottled, the fuel is burnt with a higher degree of efficiency then during operation based on the Otto principle and, furthermore, fewer NOx emissions are generated. Operation with compression ignition, however, is to be limited to partial load operation of the internal combustion engine for various reasons. On one hand, the combustion noises, which increase with increasing engine load because of the combustion pressure increase, are not acceptable for motor vehicles. On the other hand, the NOx emissions increase during combustion of lean air/fuel mixtures during compression ignition under higher loads to such an extent that a complicated and expensive exhaust gas purification becomes necessary.
Furthermore, it must be taken into consideration that the combustion chamber temperature increase required for compression ignition is achieved by retaining exhaust gas in the combustion chamber and the admixing of exhaust gases under high loads results in so-called lambda limits. In order to comply with the requirement for motor vehicle engines to cover the whole load and speed range, under high engine loads, the engine is operated under high loads based on the Otto principle. Therefore, a change-over from operation in the spark ignition mode and operation in the compression ignition mode depending on the engine operating state is necessary.
DE 199 413 A1 discloses a method of operating a four-cycle reciprocating piston engine with a changeover between compression ignition and spark ignition during full load operation with stoichiometric mixture formation wherein the exhaust gas retention for improved auto or compression ignition is to be achieved by a controlled valve opening procedure and, at the same time, blocking of exhaust gas flow by an exhaust gas flap valve. The change-over between valve overlap and non-overlap operation, that is, the switch-over between the actuating times for the gas exchange valves for compression mode operation and respectively, spark-ignition operation is achieved by displacement of a camshaft whereby the phase position of the valve opening procedure is changed. For changeover of the operating mode the known method has to determine a multitude of operating parameters, that is, the exhaust gas retention, the throttling of the intake air as well as the ignition angle in the spark ignition operating mode and the amount and time of the fuel injection and, in addition, the exhaust gas recirculation rate. All control means for the parameters to be considered during a change-over from one to the other operating mode must be brought during engine operation into tune with the respective operating mode. Since not all these many control values are controllable during a change-over of the operating mode, in the known method the change-over from one to the other mode of operation occurs over several operating cycles, wherein first the camshaft is displaced and, subsequently, the closing or, respectively, opening of the throttle valve and of the exhaust gas backup valve occurs. During the change-over, the fuel injection is to be switched off.
In a similar way, U.S. Pat. No. 6,336,436 provides during a change-over from spark-ignition to compression ignition first for a change of the phase position of the gas exchange valves for increasing exhaust gas retention in the combustion chamber in order to achieve compression ignition wherein, during compression ignition operation, the closing time of the outlet valves is retarded with respect to spark ignition operation. At the same time, the opening time of the inlet valve is advanced in order to obtain the desired valve overlap. A change-over from one to the other mode of operation occurs over several operating cycles of the internal combustion engine. Herein, the injection times of a direct fuel injection are changed so that fuel is injected during the inlet stroke while the inlet valve is closed. The particular procedure for the transfer operation is considered to be necessary in order to facilitate a long change-over operation, and to counteract knocking of the internal combustion engine during switch-over of the operating mode.
DE 101 22 775 A1 discloses a hybrid engine with alternative compression ignition and spark ignition, wherein separate camshafts are provided for the inlet valves and the outlet valves together with a variable camshaft timing control for the switch over of the control times of the gas change valves. Herein, the timing control for the two camshafts is so designed that large changes and large valve overlap can be established. In this way, during the compression ignition mode of operation, the valve overlap can be large enough so that the inlet valve opens before the outlet valve closes. The control times are adjusted herein by changing the phase positions for the opening of the gas exchange valves.
In order to provide for the exhaust gas retention needed for the compression ignition by a relatively large valve overlap as well as provide for a small valve overlap during spark ignition operation so that, by the control of the gas exchange valves, two engine operating modes can be used over the whole load and speed range of the internal combustion engine, in the known methods, a fully usable valve drive is provided which is operated mechanically, electromechanically or electro-hydraulically. Such fully variable valve drives however are very expensive.
It is the object of the present invention to provide a method of operating an internal combustion engine with a change-over between a spark-ignition and a compression ignition mode of operation by which, with little construction expenditures, a change-over between the operating modes at high engine speeds and with optimal exhaust gas emission control is obtained also with load changes occuring during the operating mode change over.